1. Field of the Invention
The present invention relates to a thin film transistor for liquid crystal display device having a structure for suppressing the deterioration of characteristics caused by a light such as a back light and the like and a liquid crystal display device provided with the thin film transistor.
2. Description of the Related Art
FIG. 5 shows an example of constitution of an equivalent circuit of an active matrix liquid crystal display device using a thin film transistor as a switch element.
In FIG. 5, a circuit is arranged such that a multiplicity of scan electrode wires G.sub.1, G.sub.2, . . . , G.sub.n and a multiplicity of signal electrode wires S.sub.1, S.sub.2, . . . , S.sub.m are wired in a matrix state and each of the scan electrode wires G is connected to a scan circuit 1 and each of the signal electrode wires S is connected to a signal supply circuit 2, respectively, thin film transistors (switch elements) 3 are disposed to the vicinities of the portions where the respective wires intersect and a capacitance unit 4 serving as a capacitor and a liquid crystal element 5 are connected to the drain of each of the thin film transistors 3.
In the circuit shown in FIG. 5, the scan electrode wires G.sub.1, G.sub.2, . . . , G.sub.n are sequentially scanned to thereby turn of all the thin film transistors 3 on one scan electrode wire at the same time and a signal charge is accumulated from the signal supply circuit 2 to the capacitance units 4 which correspond to liquid crystal elements 5 to be displayed among the capacitance units 4 connected to the turned-on thin film transistors 3 through the signal electrode wires S.sub.1, S.sub.2, . . . , S.sub.m in synchronism with the above scan. Since the thus accumulated signal charge continuously excites the corresponding liquid crystal elements 5 until the next scan is carried out even if the thin film transistors 3 are turned off, the liquid crystal elements 5 are controlled by a control signal and displayed. That is, the above drive permits the respective liquid crystal elements 5 to be driven statically even if they driven by external drive circuits 1, 2 on time sharing basis.
FIG. 6 and FIG. 7 show an example of structure in which the portions such as the scan electrode wires G, the signal electrode wires and the like are actually disposed on a substrate in the conventional active matrix liquid crystal display device shown by the equivalent circuit in FIG. 5.
In the active matrix display device shown in FIG. 6 and FIG. 7, the scan electrode wires G and the signal electrode wires S are wired on a transparent substrate 6 such as a glass or the like in a matrix state through insulation layers 9 disposed to the portions where the scan electrode wires G intersect the signal electrode wires S each other. Further, the thin film transistors are 3 disposed to the vicinities of the portions where the scan electrode wires G intersect the signal electrode wires S.
Since the thin film transistor 3 shown in FIG. 6 and FIG. 7 has a most ordinary arrangement, it includes the insulation layer 9 disposed on a gate electrode 8 provided by being drawn from the scan electrode wire G, a semiconductor layer 10 composed of amorphous silicon (a-Si), polysilicon or the like and disposed on the insulation layer 9, an etching stopper layer 7 disposed on the semiconductor layer 10, and further a drain electrode 11 and a source electrode 12 each composed of a conductor such as aluminum or the like and disposed so as to confront each other. Note, the semiconductor layer 10 is arranged as a channel portion where a carrier moves and the thin film transistor 3 shown in FIG. 7 is formed to a structure generally called an etch stopper type.
The drain electrode 11 is connected to a pixel electrode 15 formed on the substrate 6 through a contact hole 13 drilled to the insulation layer 9 as well as the source electrode 12 is connected to the signal electrode wire S. Ohmic contact layers 11a, 12a are formed under the drain electrode 11 and the source electrode 12 on the semiconductor layer 10 side thereof to establish an electric contact with the semiconductor layer 10 serving as the channel portion.
The active matrix liquid crystal display device is arranged such that a passivation layer 16 is disposed on the insulation layer 9, the drain electrode 11, the source electrode 12 and the like so as to cover them, an oriented film 17 is formed on the passivation layer 16, a transparent confronting substrate 19 including an oriented film 18 is disposed above the oriented film 17 shown in FIG. 7 and further a liquid crystal 20 is sealed between the oriented films 17, 18. Therefore, the pixel electrode 15 can control the orientation of the molecules of the liquid crystal by applying an electric field thereto. Note, numeral 22 in FIG. 7 denotes a black mask disposed to the confronting substrate 19 on the bottom surface side thereof so that it covers and conceals the portion other than the region where the pixel electrode 15 controls the orientation of the liquid crystal.
The liquid crystal display device having the above structure is usually arranged such that a polarization plate and a back light are disposed on the back side of the transparent substrate 6 as well as a polarization plate is also disposed on the back side of the confronting substrate 19 to permit a user to recognize a bright state and a dark state depending upon whether the orientation controlled liquid crystal 20 obstructs or pass the polarized state of a light emitted from the back light. However, when, for example, a light incident on the transparent substrate 6 from an oblique direction reaches the semiconductor layer 10 between the drain electrode 11 and the source electrode 12 as shown by the arrow L.sub.1 of FIG. 7, a charge is made to the semiconductor layer 10 by being excited by the light and a photoelectric current flows. This phenomenon means that a leak current flows when the thin film transistor is driven regardless of that the circuit is turned off. Since the flow of the leak current increases an off-current when the liquid crystal is driven, there is a possibility that the light permeable characteristics of the liquid crystal are adversely affected by it.
Further, when a portion of a light incident on the transparent substrate 6 from an oblique direction is reflected by the black mask 22 and reaches the semiconductor layer 10 as shown by the arrow L.sub.2 in FIG. 7, a charged is made to the semiconductor layer 10 by being excited by the light and a photoelectric current flows, thus there is a possibility that the light permeable characteristics of the liquid crystal are adversely affected by it likewise the above.
To solve the above problem, there is conventionally proposed a structure in which a source electrode 31 and a drain electrode 32 are disposed on a gate electrode 30 in confrontation with each other, the structure being arranged such that the gate electrode 30 is formed wider than a conventional one, both an etching stopper layer 33 and a semiconductor layer disposed thereunder are formed to a fallen-H shape and the etching stopper layer 33 is formed to such a width as to permit both the ends of the source electrode 31 and the drain electrode 32 to reach the half portions of the projections 33a of the etching stopper layer 33, as a plan structure is shown in FIG. 8A.
According to the structure of the example, since it is found that when the above photoelectric light flows, it flows through the side portion of the semiconductor layer located under the fallen-H-shaped projections 33a of the etching stopper layer 33, the photoelectric current is suppressed by increasing the length of the conductor bus of the photoelectric current at the side portion in the structure of the example.
Further, as shown in FIG. 8B, there is proposed a structure in which a source electrode 36 and an L-shaped drain electrode 37 are disposed on a gate electrode 35 in confrontation with each other, the structure being arranged such that both an etching stopper layer 38 and a semiconductor film thereunder are formed to an inverted-C-shape and the etching stopper layer 38 is formed to such a width as to permit both the ends of the source electrode 36 and the drain electrode 37 to reach the half portions of the projections 38a of the etching stopper layer 38. A photoelectric current is suppressed by increasing the length of the conductor bus of the photoelectric current at the side portion of the semiconductor layer also in the structure of this example.
In the structures shown in FIGS. 8A and 8B, however, since the increase of the length of the conductor bus of the photoelectric current in the semiconductor layer means that the area of the portion to be covered and concealed by the black mask 22, that is, the area of the portion including the gate electrode, the etching stopper layer, the semiconductor layer, the drain electrode and the source electrode which is different from the region where the orientation of the liquid crystal is controlled is increased, there arises a problem that an numerical aperture is lowered as a liquid crystal display device and a bright display is difficult to be obtained.
An object of the present invention made taking the above circumstances into consideration is to provide a thin film transistor for liquid crystal display device arranged such that a light is difficult to be incident on the portion of a semiconductor layer from a back light and the like so as to suppress a leak current in the semiconductor layer to thereby lower the off-current of the thin film transistor and even if the light is incident on the semiconductor layer, the leak current is made difficult to flow to thereby lower the off-current and a liquid crystal display device including the thin film transistor.